Micromechanical sensors have witnessed rapid development owing to research of several decades. Among micromechanical sensors, the common ones include micromechanical accelerometers and micromechanical gyroscope.
Among others, accelerometer is a device used to measure acceleration imposed on objects. High-precision accelerometers are one of key basic elements as well as an integral part in inertial navigation systems. Accelerometers have many varieties, including liquid floated pendulous ones, flexibility pendulous ones, vibrating string ones, pendulous integrating gyro ones and so on. Micromechanical accelerometers are developed based on micro-electronic industry, which can be integrated on the same chip with sensitive circuits. Owing to such advantage as small volume and low cost, micromechanical accelerometers are widely applied in automobile and consumer electronics fields.
Gyroscopes are sensors used for measurement of angular velocity and displacement, which is a key part of inertial measurement unit. Gyroscopes have witnessed a development course of more than one hundred years since the initial demonstration of self-rotation of the earth with the help of gyroscopic inertia of a high-speed rotating rigid body. Rotary, optical and micromechanical gyroscopes have made their appearance in succession. Coriolis force serves as the sensitive mechanism for vibrating micromechanical gyroscopes. Coriolis force is in direct ratio with rotating speed of objects. The rotating speed of objects can be obtained from measurement of Coriolis force.
The measurement types of micromechanical sensors can be divided into piezoresistive, capacitive, vibration and tunnel types. Among them, capacitive micromechanical sensors have become the mainstream design due to their quick response and simple fabrication.
Specifically, capacitive micromechanical accelerometers can be divided into two types, variable-gap and variable-area capacitor structures. A conventional comb grid capacitor accelerometer has a variable-area capacitor structure. However, because its movable and fixed electrodes are in rectangular forms (according to conventional design), its elasticity coefficient cannot be adjusted.
Micromechanical capacitive gyroscopes also fall into variable-gap and variable-area capacitor structures. Micromechanical gyroscope is associated with frequency difference in driving and sensing directions in terms of such important performance indicators as sensitivity and bandwidth. Reasonable frequency difference should be selected properly in design phase to realize performances as required. It is normally difficult to obtain designed frequency difference during manufacture of micromechanical gyroscopes due to limited manufacturing precision and consistence. Consequently, performance of elements may not be up to the design standard. Normally, it is necessary to proceed with adjustment to resonance frequency, namely electric tuning, in driving or sensing directions so as to improve performance of gyroscopes. It has been applied to gyroscope elements with variable-gap capacitor structures. With regard to existing variable-area capacitor structure, it is unlikely to improve performance of comb grid capacitor gyroscope effectively due to failure to adjust the resonance frequency of comb grid capacitor gyroscope in driving and sensing directions.